The goals of this application are to address the development of CD40+ T cells, the role that CD40 plays in T cell receptor (TCR) revision and how induced TCR revision impacts autoimmunity. CD40 has a direct role in autoimmune diseases including type 1 diabetes (T1D). We discovered that a unique subset of effector T cells, described as CD4loCD40+, are highly pathogenic in T1 D. CD40 is present on diabetogenic T cell clones and primary CD4loCD40+ T cells expand concurrently with insulitis in NOD mice. Importantly, CD4loCD40+ T cells readily transfer T1D to NOD.scid recipients. Mechanistically, CD40 induces the RAG1/RAG2 recombination machinery in T cells. Following that induction, TCR revision, induced alteration of TCR expression, occurs. Some revised primary T cells were found to be highly pathogenic. However, CD40 induced TCR revision could serve as a functional tolerance mechanism of established diabetogenic T cell clones. Specifically, CD40 engaging the diabetogenic T cell clone BDC2.5 severely ablated disease transfer ability of the clone. We showed that blocking CD40 - CD154 (the ligand for CD40) interaction in NOD mice at 3-weeks of age prevents not only T1D onset, but prevents expansion of CD4loCD40+, autoaggressive cells. In this application we will explore how CD40 affects the unique T cell population. Our hypothesis is that TCR revision directly impacts T cell tolerance. Specific Aim #1: Do CD4loCD40+ T cells comprise a unique T cell subset or represent a transient activation state within T cells? If CD40 is induced on T cells are there differences in those T cells compared to the intrinsic CD40 T cell subset? We plan experiments to address this central concern. Specific Aim #2: How does CD40 engagement affect TCR revision in CD4loCD40+ T cells? Are there differences in response to CD40 engagement between autoimmune and non-autoimmune conditions? We will explore RAG1 and RAG2 induction, kinetics of TCR revision and how different sources of CD154 in autoimmune versus non-autoimmune mice affect this process. Specific Aim #3: How does CD40 engagement affect T cell antigen specificity? We will explore how induced TCR revision affects T cell antigen specificity using in vitro and in vivo models. These proposed studies address not only the development of potential autoaggressive T cells but will provide important information as to whether induced TCR revision is pathogenic or tolerogenic.